1. Field of the Invention
This invention relates to a method for enhanced recovery of petroleum from a subterranean oil-bearing formation. More particularly, this invention relates to secondary or tertiary recovery of oil employing a polymer thickened aqueous drive fluid. The polymeric viscosifier for the drive fluid is selected from a class of hydrophobically associating water soluble polymers containing one or more water soluble monomers and a water insoluble monomer or group. The water soluble groups are acrylamide (AM) and N-vinyl pyrrolidone (NVP) and a salt of acrylic acid (H) and the water insoluble group is a higher alkylacrylamide (R). These polymers, hereinafter referred to as NVP-HRAM, when added to an aqueous brine solution have the ability to substantially increase the viscosity of the solution. The control of displacement fluid mobility results in more uniform sweep efficiency and improved oil recovery. In addition, aqueous solutions of these hydrophobically associating polymers exhibit enhanced viscosification, reduced salt sensitivity, improved thermal stability and other desirable properties found useful in chemically enhanced oil recovery processes.
2. Description of the Prior Art
The recovery of petroleum from oil-bearing formations initially involves drilling into the reservoir and utilizing the natural pressure for production. However, these primary production methods recovery only a minor portion of the oil present in the reservoir. To further improve the amount of oil recovered, a variety of techniques have been employed. These include miscible techniques, such as gas flooding; thermal methods, such as heating or steam injection; chemical methods, such as surfactant/polymer or alkaline injection; and water flooding processes. While these methods address the need to reduce oil viscosity, reduce oil-brine interfacial tension, or repressurize the formation, many deficiencies still exist which limit the amount of additional oil recovery. For example, a well-known limitation involves the poor sweep efficiency caused by the drive or displacement fluid having a higher mobility than the oil-in-place. This results in an instability manifested by viscous fingering of the drive for displacement fluid through the oil and a reduction in recovery efficiency.
It has long been known that water soluble polymers can be added to the drive water to increase viscosity or reduce mobility and thus improve the sweep efficiency and overall oil recovery. Polyacrylamide (PAM) and partially hydrolyzed polyacrylamide (HPAM) are well known water soluble polymers used as flocculation aids for waste water treatment and dewatering sludge, and for rheology control for secondary or tertiary oil recovery. Further examples of the properties and use of these polymers can be found in Handbook of Water Soluble Gums and Resins, R. L. Davidson, Ed., McGraw Hill 1980, Chapter 16 by H. Volk and R. E. Friedrich or in Water-Soluble Polymers, N. M. Bikales, Ed., Plenum Press, 1973, by D. C. MacWilliams, J. H. Rogers and T. J. West. The use of these polymers for secondary recovery of petroleum has been described by Sandiford and Keller in U.S. Pat. No. 2,827,964. Kolodny, U.S. Pat. No. 3,002,960, describes a method for preparing higher molecular weight PAM based on redox initiation. McKennon, U.S. Pat. No. 3,039,529, describes the importance of controlling the extent of hydrolysis or the amount of anionially charged carboxyl groups to minimize adsorption during secondary recovery of petroleum with these HPAM polymers. A series of improvements in HPAM polymers for secondary oil recovery are described in U.S. Pat. Nos. 3,087,543, 3,308,885, 3,721,295, 3,724,547, 3,779,316, 3,780,806, 3,893,510, and 4,034,809.
Polyacrylamides rely on a combination of high molecular weight and chain expansion due to repulsion of pendent ionic groups along the polymer chain to viscosify or thicken aqueous fluids. However, high molecular weight polymers mechanically degrade when subjected to large elongational or shear stresses such as found in pumps or during injection into reservoir rocks. This degradation results in permanent reduction in polymer molecular weight and in turn loss of viscosification efficiency. Gogarty, U.S. Pat. No. 3,580,337, suggests the use of water-soluble salts composed of divalent cations to stabilize HPAM solutions with regard to mechanical degradation. However, subsequent studies by Maerker (Soc. Pet. Engr. J., 1975) indicate that the presence of divalent cations such as calcium leads to increased mechanical degradation during injection into porous media. The presence of cations in aqueous solution, in particular divalent cations, shields the ionic charged groups on the polymer. This causes the polymer chains to collapse into a random coil configuration having a smaller hydrodynamic volume, and thereby losing significant viscosification efficiency. Thus, polymeric viscosifiers based on an alternative mechanism of viscosification providing improved mechanical stability and salt tolerance relative to PAM and HPAM polymers would be highly desirable.
The use of alternate polymer structures to overcome the deficiencies in polyacrylamide systems has been explored. For example, Norton, U.S. Pat. No. 3,747,676, describes an oil recovery process using a methylolated unhydrolyzed polyacrylamide while Morduchowitz, U.S. Pat. No. 4,323,463, describes terpolymers of acrylamide, acrylonitrile and acrylic acid. In the area of anionic sulfate or sulfonate containing systems, Kaufman, U.S. Pat. No. 3,679,000, uses N-sulfohydrocarbon substituted acrylamide polymers. Hunter discloses in U.S. Pat. Nos. 4,226,730 and 4,226,731 secondary recovery processes using a water soluble ethoxylated polyphenol which is sulfated or sulfonated; and in U.S. Pat. Nos. 4,338,203 and 4,343,712 he describes copolymers of acrylamide with vinyl sulfonic acid and styrene sulfonic acid alkoxylated to various extents. None of these patents describe solution rheological properties differentiating or distinguishing these polymers from the prior act. More recently water soluble copolymers of acrylamide and sulfonated monomers have been studied as aqueous fluid viscosifiers. For example, C. L. McCormick and G. S. Chen, J. of Polymer Science: Polymer Chemistry Ed., Vol. 20, 817-838 (1982) describe the synthesis and characterization of random copolymers of acrylamide and sulfonated monomers such as sodium-2-sulfoethyl methacrylate or sodium-2-acrylamido-2-methylpropane sulfonate. In a recent paper on the dilute solution properties of these polymers, H. H. Neidlinger, G. S. Chen and C. L. McCormick, J. of Applied Polymer Science, Vol. 29, 713-730 (1984) noted the high salt sensitivity of these polymers, particularly for copolymer compositions containing more than about 25 mole percent sulfonate monomer.
Processes for preparing polyacrylamides are well known in the art; Tanaka et al., U.S. Pat. No. 4,154,910 teaches an aqueous solution method using the heat of polymerization to concentrate the product. Zimmerman et al., U.S. Pat. No. 3,211,708 teaches an oil-in-water bead polymerization for polymerizing water soluble monomers. These techniques result in moderate molecular weight polymers exhibiting poor viscosification efficiency particularly in saline solutions. Kolodny, U.S. Pat. No. 3,002,960 teaches a low temperature, redox initiated solution polymerization resulting in high molecular weight polyacrylamide. Another approach to high molecular weight water soluble polymers is described by J. W. Vanderhoff et al., U.S. Pat. No. 3,284,393, where water soluble monomers are polymerized at high concentration in a water-in-oil emulsion. While some of these processes allow high molecular weight polymers to be prepared, the resulting PAM and HPAM systems provide only fair viscosification efficiency, poor mechanical stability and low salt tolerance.
N-vinyl pyrrolidone (NVP) homopolymers are well known and reviewed by Davidson and Sittig in Water-Soluble Resins. Such homopolymers are characterized by good hydrolytic stability, even in the presence of electrolytes, but poor viscosification efficiency (viscosity per unit concentration) and undersirable adsorption on rock. (H. P. Frank, J. Polymer Sci., 12, 5;65 (1954); A. Conix, J. Polymer Sci., 15, 221 (1955); G. A. Stahl, European Patent Application No. 84100918.6 (Jan. 30, 1984).
Copolymers of NVP with acrylamide (AM) have also been disclosed. (A. M. Chatterjee and C. M. Burns, Canadian Journal of Chem., 49, 3249 (1971); G. A. Stahl, European Patent Application No. 84100918.6 (Jan. 30, 1984). Such materials have improved hydrolytic stabilities. However, the viscosification efficiency of such materials is still low.
G. A. Stahl (European Patent Application No. 84100918.6, Jan. 30, 1984) broadly teaches terpolymers of NVP, AM and minor amounts of a third monomer selected from the group of hydrophobic compounds as vinyl pyridines, hydroxylated esters of ethylenically-unsaturated carboxylic acids and N,N-alkylacrylamide, where the alky group contains more than 2 carbon atoms. (Stahl, page 23, line 15 to page 24, line 25). While these monomers are hydrophobic, some are water dispersible or even water soluble (e.g., isopropyl acrylamide) and some are water insoluble (e.g., n-octylacrylamide). Thus, the critical distinction between water soluble (dispersible) hydrophobic monomers and water insoluble hydrophobic monomers was not made. Moreover, no terpolymers containing the highly hydrophobic (insoluble) long chain N-alkylacrylamide were exemplified, possibly because of the difficulty in incorporating such monomers into homogeneous water soluble terpolymers.
One approach to overcoming the deficiencies in these polyacrylamide based systems is described by Turner et al., U.S. Pat. No. 4,520,182. Water soluble acrylamide copolymers containing a small amount of oil soluble or hydrophobic alkyl acrylamide groups were found to impart efficient viscosification to aqueous fluids. Furthermore, since these alkylacrylamide-acrylamide copolymers (RAM) were nonionic, they were relatively insensitive to the level of salt in the water. However, these polymers required concentrations above about 2000 ppm to provide significant viscosification. Landoll, U.S. Pat. No., 4,304,902 describes copolymers of ethylene oxide with long chain epoxides which also required relatively large polymer concentration (approx. 1%) for thickening water and required surfactants for solubility due to irregularities in the polymerization. In a related case, U.S. Pat. No. 4,428,277, modified nonionic cellulose ether polymers are described. Although these polymers show enhanced viscosification relative to polymers not containing hydrophobic groups, the viscosification efficiency was very low, and required 2 to 3 weight percent polymer to provide an enhancement. The use of surfactants to enable solubility and in turn viscosification by a water soluble polymer containing hydrophobic groups is described by Evani, U.S. Pat. No. 4,432,881. The hydrophobic group claimed is attached to the polymer via an acrylate linkage which is known to have poor hydrolytic stability. In addition, the need for a surfactant to achieve solubility and thickening efficiency should make such a system very salt sensitive as well as very sensitive to small changes in surfactant and polymer concentration. Emmons et al., U.S. Pat. No. 4,395,524 teaches acrylamide copolymers as thickeners for aqueous systems. While these polymers possess hydrophobic groups, they are prepared using alcohol-containing solvents which are known chain transfer agents. The resulting polymers have rather low molecular weights and thus relatively high polymer concentrations are required to achieve reasonable viscosification of water based fluids.
One of the objects of this invention is to overcome the deficiencies in the use of the water soluble polymers of the prior art for secondary or tertiary oil recovery operations. The present invention uses a new class of water soluble polymers described by Schulz et al. in copending application U.S. Ser. No. 814,362, now U.S. Pat. No. 4,663,408 and is incorporated herein by reference. These polymers have been found to impart enhanced viscosification to aqueous fluids, improved mechanical and hydrolytic stability and better salt tolerance, characteristics highly desirable for secondary and tertiary oil recovery. These polymers contain a nonionic water soluble monomer such as acrylamide (AM), and N-vinyl pyrrolidone (NVP), an anionically charged water soluble monomer such as an alkali metal acrylate (H) and a water insoluble or hydrophobic monomer such as an alkylacrylamide (R) with a chain length of six carbons or greater. When these polymers, hereinafter referred to as NVP-HRAM tetrapolymers, are dissolved in an aqueous solvent, the hydrophobic groups aggregate or associate in a manner similar to a surfactant. This hydrophobic association between polymer chains in solution results in an increase in the effective hydrodynamic size of the molecule which in turn causes an increase in solution viscosity. The presence of ionic groups, such as sodium acrylate, cause an expansion of the polymer in solution, an improvement in polymer solubility and a favorable influence on the association of the hydrophobic groups. Thus, polymers containing both ionic acrylate groups and hydrophobic groups provide a significant improvement in viscosification efficiency of water based systems and other properties useful for secondary and tertiary oil recovery.
Synthesis of the hydrophobically associating polymers used in the secondary or tertiary oil recovery process of the instant invention presents difficulties. The incompatibility of the oil soluble and water soluble monomers prevents an effective concentration of one or the other of these monomeric species from being achieved at the locus of polymerization of the other comonomer. Techniques for polymerizing water soluble polymers such as those taught in U.S. Pat. Nos. 4,154,910, 3,211,708, 3,002,960 and 3,284,393 cannot be used to prepare the compositions of this invention. This art does not teach the formation of a sufficiently fine dispersion of the water and oil soluble monomers to enable uniform reaction and efficient aqueous viscosifiers to be prepared. The use of mutual solvents or solvent mixtures to dissolve the water and oil soluble monomers as taught by Lenke et al., U.S. Pat. No. 4,151,333 and Barua et al., U.S. Pat. No. 4,098,987 also has some serious limitations. Although this approach undoubtedly allows the incompatible monomers to come into close proximity to one another, since the dispersion is on a molecular scale, often the resulting copolymer is insoluble in the same solvent as shown in U.S. Pat. No. 4,151,333. This leads to precipitation of the copolymer before it has achieved sufficient molecular weight to provide efficient aqueous viscosification. The use of water miscible solvents such as alcohols, ether and acetone either alone or with water as taught in U.S. Pat. No. 4,098,987 results in extremely low molecular weight (e.g. 10,000) polymers due to the high chain transfer characteristics of these solvents. Thus, polymers produced by these teachings are rather ineffective viscosifiers for aqueous fluids. One technique found useful for preparing the hydrophobically associating polymers used in the secondary or tertiary oil recovery process of this invention was based on dispersing the oil soluble monomers using an aqueous micellar solution of the water soluble monomers. Suitable surfactants and the details of the polymerization are taught by Bock et al. in U.S. Pat. No. 4,528,348 which is incorporated herein by reference. Using the micellar polymerization technique, there are two different routes for preparing the terpolymers of this invention. The first involved preparation of the alkylacrylamide-acrylamide, N-vinyl pyrrolidone terpolymer by the micellar polymerization followed by controlled hydrolysis of some of the acrylamide groups to anionically charged metal acrylate groups. An alternative technique involved the tetrapolymerization of acrylamide, N-vinyl pyrrolidone, alkali metal acrylate, and an N-alkylacrylamide preferrably using the micellar polymerization technique. Further details of the preparation of the tetrapolymers can be found in copending application, U.S. Ser. No. 814,362, now U.S. Pat. No. 4,663,408. It is an object of this invention to improve the viscosification efficiency of brine solutions used for secondary and tertiary oil recovery and thus provide an improved process for the recovery of oil from subterranean formations. It is a further object to improve the salt tolerance, hydrolytic and mechanical stability of brine drive solutions used for mobility control during secondary and tertiary oil recovery operations. Yet another object of this invention is to provide a water soluble additive for use in rheological control during secondary and tertiary oil recovery operations.